Wandering Traders are not spawning with backpacks. I've only really tried to do this using eggs and the summon command. On my mod pack I'm only getting the base games version and Alex's Mobs Elephant Trader.

Forge Version : 1.18.2 - 40.1.73
Backpacked : backpacked-2.1.10-1.18.2

Oi! I figured it out. The Fresh Animations Resource Pack is making the backpack invisible or it's just not there at all because the pack is using a different model and overriding it.